1. Field of Invention
The present invention generally relates to a method and apparatus for manufacturing composite parts from a plurality of individual plies of thermoplastic resin, prepregs, enclosed in a vacuum bag which forms an inner chamber that is surrounded by an outer chamber, and more particularly to such a method and apparatus wherein said composite parts are manufactured with minimum porosity and essentially wrinkle free by controlling the flow of gas from the inner and outer chambers to assure that the pressure differential therebetween, if any, is kept as small as possible and the rate of change of any pressure differential between the inner and outer chambers is kept as small and as gradual as possible.
2. Description of Prior Art
Typically, shaped composite parts are made from commercial, fiber reinforced prepregs containing polymeric thermoplastic resins. During the manufacture of composite parts, a plurality of plies of fiber reinforced thermoplastic prepregs are placed on a shaped tool or mold in an uncured, pliant state. These parts are then processed using a well known vacuum bag molding technology wherein the plurality of plies on the shaped tool are covered with a flexible, porous breather material and this "lay-up" is then covered with a hermetically sealed flexible vacuum bag. The assembly is then evacuated in an effort to remove as much contaminant, such as air, water vapor, and volatile as possible. The bagged lay-up is then subjected to high temperature and/or pressure while maintaining a vacuum to cure the part. Presently autoclaves are required to apply the needed temperature and pressure to produce acceptable parts. Autoclaves are large, expensive pieces of equipment and cannot always provide void free laminates. Voids are usually caused by the inability of the resin to displace air during the time the fibers are coated with the liquid resin. Voids are also caused by air bubbles entrapped during the lay-up operation. Voids can also be caused by entrapment of air, moisture, and resin volatile at the time of consolidation. Moreover, the size, weight and cost of an autoclave prevents it from being used in field maintenance locations or smaller manufacturing facilities.
Attempts have been made to eliminate the expensive autoclave from the process. U.S. Pat. No. 4,357,193 to McGann et al. (1982) illustrates one such attempt; however, a mating rigid vacuum chamber is required to accomplish the McGann et al. process. The expense of such a vacuum chamber for each part shape makes this process very expensive. U.S. Pat. Nos. 5,116,216 (1992) and 5,236,646 (1993) to Cochran et al. disclose another attempt to eliminate the expensive autoclave process; but this process also requires the expensive concentric vacuum chamber. Although parts with adequate interlaminar shear strength yielding the desired structural integrity can be manufactured by these methods, other problems are inherent in these processes. Undesirable wrinkles in the part can make them unusable, requiring expensive remanufacture. In the manufacture of aircraft parts, for example, rejection rates of around 20% have been recorded.
Wrinkles appear in the part when a large pressure differential is used between the bag vacuum (or inner chamber vacuum) and the outer chamber vacuum. These wrinkles can be minimized only by the use of the expensive concentric chamber to limit the movement of the lay-up in these processes. A matched tool or concentric chamber is required for each and every part shape. Placing the concentric chamber over the bagged lay-up can damage the bag and/or the part due to the tight tolerance of tool to part. The rigid chamber is used to seal and restrain the bag, leading to increased labor cost. All of the above referenced patents require the use of a breather material to aid in the evacuation of the lay-up. This is an added cost in both material and labor. The process described by McGann et al uses two vacuum bags resulting in increased labor and material costs. In addition, said processes require two separate vacuum sources at additional expense.
Previous processes have failed to take advantage of the higher vacuums available from standard oil sealed vacuum pumps. Typically, as described by both McGann et al and Cochran et al , vacuums are limited from about 26 to 28 inches of mercury (in Hg) vacuum or 97 to 49 mm Hg absolute pressure. Standard oil sealed vacuum pumps can supply end vacuums of 29.9 in Hg vacuum or less than 1 mm Hg absolute pressure. These referenced processes use vacuums which do not readily remove contaminants such as water vapor and volatile at room temperatures. At room temperature, the vapor pressure of water is approximately 20 mm Hg. absolute. Volatile vapor pressures are around 5 mm Hg absolute. In order for a 28 in Hg vacuum to remove unwanted contaminants, such as water vapor and volatile, the temperature must be increased which advances the resin toward its cured state.
Normally, fiber reinforced thermoplastic resin prepregs are stored frozen in order to prevent them from advancing toward a cured state thereby increasing their shelf life. Frozen storage often causes water vapor to be trapped in the prepreg, contaminating the lay-up and lowering the interlaminar shear strength. Use of pressures less than 20 mm Hg promotes removal of this contaminant at room temperature and does not advance the resin toward its cured state. U.S. Pat. Nos. 5,116,216 and 5,236,646 both state that theoretically, it is advisable to maintain as high a vacuum as possible in the inner chamber, yet they limit vacuums to about 28 in Hg which is not sufficient to remove water vapor and most volatile at ambient temperatures. U.S. Pat. No. 4,421,589 to Armini et al (1983) and U.S. Pat. No. 4,944,589 to Ishikawa et al (1990) discuss the advantages of using process vacuums of around 1 mm Hg absolute but these references are not concerned with the production of fiber reinforced prepregs.
Autoclaves and ovens use heated gases and forced convection to provide the temperature needed to soften the resin prior to consolidation of the composite laminate. "Hotspots" can develop within the lay-up causing the resin in some areas to advance more rapidly toward cure than resin in other areas. The ideal method of heating a lay-up would be to evenly heat only the plurality of prepregs. Since it would be difficult to accomplish this goal, the best approach is to heat only the lay-up and tool. Heating elements placed under the tool was demonstrated by McGann et al but this method does not take into account the various thickness necessary in many lay-ups. Heated blankets have also been used as in Cochran et al but, again, this method does not take into account the various thickness of a lay-up.